Circuit for controlling transmission in signaling systems



Sept. 10, 1935. S B. s. BJORNSON ET AL 2,014,223

CIRCUIT FOR CONTROLLING TRANSMISSION IN SIGNALING SYSTEMS Filed July 28, 1935 2 Sheets-Sheet l 27 \25 2:? EA I )(AMP java/25R 32 l Hmli D/SABLER &

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GAS FILLED SUPPRES'SOR 8. a. BJORMSON Q. .GREENWOOD ATTORNEY SUPPRESSOR p 1935. B. s. BJORNSON El AL 3 CIRCUIT FOR CONTROLLING TRANSMISSION IN SIGNALING SYSTEMS Filed July 28, 1935 2 Sheets-Sheet 2 66 l 30 AMP EQUAL/Zak D/SABL ER OPERATOR TUBE GAS FILLED FIG. 2

E'OUALIZEIL/B B. c. womvso/v VENTORSQ. E. GREENWOOD A T TORNEV Patented Sept. 10, 1935 NETED STATS ATET QFFIQE CIRCUIT FOR CONTROLLING TRANSMIS- SION IN SIGNALING SYSTEMS Application July 28, 1933, Serial No. 682,580

9 Claims.

The present invention relates to circuits for controlling transmission in a signal transmission system, and particularly to signal-controlled cir-' cuits for suppressing echoes and preventing singing in two-way signal transmission systems.

An object of the invention is to improve the operatin characteristics of signal-controlled circuits for suppressing echoes and preventing singing in two-way signal transmission systems.

Signal-controlled, circuit-control apparatus, commonly called echo suppressors or anti-singing devices, are usually employed in connection with the repeating circuits of along two-way signaling system, to insure that the system transmits in only one direction at a time so as to suppress echoes and prevent singing. This is usually accomplished by making the circuit-control apparatus responsive to signal transmission in either direction over the system to disable the transmission path for the opposite direction by increasing the attenuation therein, or to render operative the transmission path for the direction of signal transmission by reducing the attenuation therein, or to do both.

Certain circuits of the above mentioned type in the prior art have been designed to operate in response to transmitted signals to properly control the attenuation in the transmission paths by varying the output impedance of a vacuum tube or tubes connected effectively in parallel with the repeating paths. By this means the attenuation in the repeating path transmitting signals is made or maintained small so that it transmits the signals freely, while the attenuation of the oppositely directed repeating path is increased to the point where disturbing echoes or reflected currents are substantially suppressed. A difficulty encountered in prior art echo suppressor 40 circuits of this type is the production of unbalance currents or transients in the transmission paths due to sudden changes of impedance or loss therein when the echo suppressor operates. These transients may result in annoying click 45 disturbances in the receiving apparatus of the system, their disturbing efiects being enhanced by the long time delay in very long transmission circuits.

In the echo suppressors of the invention sub- 50 stantial elimination of click and other disturbances as well as fast operation, a high degree of suppression and uniform hangover in operation are attained by suitably designed circuits utilizing gas-filled tubes. The objects and advantages of the invention will be clear from the following detailed description thereof when read in connection with the accompanying drawings Figs. 1 and 2 of which show schematically portions of a four-wire toll telephone system equipped with echo suppressors embodying different modifications of the invention.

The four-wire toll telephone circuit of Fig. 1 comprises a one-way transmission path EA for repeating telephone signals in the direction from West to east and a one-way transmission path WA for repeating telephone signals in the direction from east to west. The transmission paths EA and WA may be connected at their ends in substantially conjugate relation with each other and in energy transmitting relation with the twoway lines between which the telephone signals are to be repeated, by means of the usual hybrid coil transformers and associated balancing networks (not shown) or by any other suitable means.

As indicated in Fig. l, the echo suppressor associated with the transmission paths EA and WA, comprises a suppressor portion and a disabler portion. The suppressor portion of the echo suppressor'comprises three parts; the input circuit, the control circuit and the variable impedance or suppressing element.

The input circuit furnishes the required amplification and selective action. It comprises, as indicated in Fig. l, a special high input impedance amplifier I having its input connected across the path WA, the suppressor amplifier 2 and a selective network or equalizer 3 coupled between the ouput of the amplifier I and the input of the suppressor amplifier 2. The equalizer 3 may be any network which will give the proper input frequency-sensitivity characteristic to the suppressor to make it more selective to speech than noise. This may be accomplished, for example, by designing the equalizer so that it produces the least loss to the frequencies which predominate in speech.

The suppressor amplifier 2 comprises a threeelectrode thermionic amplifying tube (for example a Western Electric Company 2llA tube) having a low negative grid bias and a large resistance 4 in the grid circuit in series with the secondary of the input transformer 5. This is for preventing an excessive increase in the hangover of the suppressor at high input levels.

The output of the suppressor amplifier tube is coupled to the control circuit by means of transformer 6. The control circuit comprises the GQntrol gas-filled tube 1 comprising a cathode 8, an .anode or plate 9, a cylindrical grid It! located between the cathode and plate, and a spiral grid Ii. The spiral grid II is located between the cathode 8 and the cylindrical grid I 0, being very close to'the cathode. The spiral grid I I corresponds to the control grid of the ordinary three-electrode thermionic tube being connected to the cathode 8 throughthe secondary of the input transformer 6 and the negative grid bias- 7 ing battery I2. The cylindrical grid I is connected directly to the cathode 8. Under these conditions the cathode 8 and the cylindricalgrid I0 V will be at the same potential and whenthe impressed input voltage is sufiicient to produce ionization in the tube, the. positive ions will, in general, move toward both the cathode and cylindrical grid, but the cylindrical grid will collect a large fraction of the positive ions, due to its large surface area. The space between the cylindrical grid and cathode will then contain relatively few positive ions; thus tending to leave the spiral grid in control of the cathode emission and hence of the plate current.

The variable impedance element of the suppressor comprises .a four-element gas-filled tube I3, similar to the gas-filled control tube 1, having a cathode I4, an anode or plate I5, a cylindrical grid I6 located between the plate and cathode, and the spiral grid I! located between the cathode and cylindrical grid andnear, the cathode. The

' plate-cathode circuits of the control. tube I and the variable impedance tube .I 3 are in series. The resistance I8 in this series circuit is provided to limit the plate currenttoasafe value. Thehighresistance !9 connected in'series with the-plate battery 20, choke coil M and resistance I8 between the cathode and plate of the control tube 1,,is shunted between the plate I andthe cathode I4 of the variable impedance tube I3 so as tonortrol in the control't'ube, while the other combination comprising a. larger resistance 23 and a larger condenser 24' in series having'a longer time constant being provided to smooth out the plate current in the variable impedance tube "I3, and to give the proper hangover time to the suppressor. The variable impedance path between the cylindrical grid I8 and the cathode I4 of the variable impedance tube I3 is shunted across the path EA at the points 25, 26 between the step-up transformer 21 and the step-down transformer 28. The transformers are provided for the purpose of stepping-up the impedance of the portion of the path EA therebetween and thus enabling 1 a higherloss to be inserted by the variable impedance tube I3 inthat path when the tube is operated in the manner which will be described below. The spiral grid ll of the tube I3 is norjmally maintained bythe grid-biasing battery 44 at a potential sufficiently below that of, the cathode I4 to prevent the setting up of ionization in the tube I 3 by normally transmitted speech currents in the path EA.

The input circuit of the disabler portion of the suppressor is connected across the path EA on the low side of the step-down transformer 28. It comprises the special high input impedance amplifier 30, similar to the amplifier I in the suppressor, having its input connected across the path EA, the disabler amplifier 3!, similar to the 5 suppressor amplifier 2, and the selective network or equalizer 32 coupling the output or" the amplifier 39 to the input of the amplifier 3|. The

equalizer 32 is designed to give the disabler circuit the same input frequency-sensitivity characteristic'as given to the suppressor circuit by the selective network or equalizer Etherein.

The output of the disabler amplifier 3! is coupled by transformer 33 to the input of a three-' electrode, gas-filled tube 34 (for example, a Western Electric Company 256-A tube) which will be referred to hereafter as the disabler tube; V The tube34 comprises a cathode 35, an anode orplate 36 and a grid or control electrode 3?. The cylindrical grid 31 of the tube 34 is negatively biased by grid battery 38. Connected directly across the plate 36 and the cathode 35 of the disabler tube 34 is a resistance 39 in series with a condenser 40. Plate current is supplied to the plate 35 of disabler tube 34 from plate'battery 20 through choke coil 4|, high resistance 42 and choke coil 43. The high resistance 42 is provided to keep the current in the plate circuit of the disabler gas-filled tube 34 at a low value so as to enable control of the space current there- 3 in by the grid 31. When plate current flows in disabler tube 34, additional negative bias is supplied to the spiral grid I! of the variable impedance tube I3 by the IR drop in resistance 42. Q The resistance-condenser filter 45 smoothes out fluctuations in this additional bias.

.The operation of the suppressor will now be described- With no signal currents flowing in path WA, the impedance between the cylindrical grid Ifiand the cathode I4 of variable impedance tube I3, connected across the path EA, is substantially infinite and the loss inserted in the path EA by that tube is zero or .very low. The path EA is therefore in condition to transmit efiiciently in the direction from west to east. 7

Let it be assumed that speech waves are being transmitted over the path WA in the direction from east to west and, at the time these waves arrive at the input to the suppressor, no'westto east currents have arrived in the path EA at'the point at which the input of the disabler is connected. A portion oi'these speech waves in the path WA will be diverted into the suppressor circuit and will be selectively amplified in the input circuit thereof. The amplified speech currents in the output of the suppressor amplifier -2 are impressed upon the input of the gas-fi1led control tube I by transformer 6.

When the speech input voltage impressed upon the control tube I reaches a definite value ionization takes place in the tube and current will pass from its plate 9 to its cathode 8. This is followed practically instantaneously by the establishment of a flow of current between the cathode I4 and the plate i5 of the variable impedance tube I3 due tothe voltage drop in the resistance I9 connected therebetween. This flow of current in the plate circuit of the variable impedance tube I3 establishes a relatively low impedance between cylindrical grid iii and the cathode I4 of that tube which, by its shunting action across the high impedance portion of the path EA, produces high loss therein, and efiectively disables the path EA.

While the speech voltages are being applied to the control tube 1, the plate currenttherein is very irregular going on and ofi at a rate de-' pending on the input level. Before ionization starts in the tube, condensers 24 and 22 are charged to the voltage of the plate battery 20, but when a positive peak of the applied speech voltage starts ionization in the tube the large condenser 24 will discharge through the resistance 23 in series therewith and tube 1 and the small condenser 22 will discharge through the resistance 2| and the tube to a voltage equal to the voltage drop across the tube. The small shunt condenser 22 and the small series resistance 2i are provided to make the control tube 1 unstable so as to enable high frequency oscillations therein to be quickly started by an applied input voltage when the current through the tube falls to the value at which the spiral grid of the tube regains control.

When the current from the discharging con densers decreases to such a value that the negative voltage swing of the input waves becomes sti fiicient to stop ionization in the tube, the spiral grid II in the tube 1 will regain control of the space current. As soon as ionization stops, the condensers 22 and 24 will charge up again. The next positive peak of the input speech voltage will start ionization again and the process will be repeated. Thus, the plate currentin the variabl impedance tube I3 is maintained by the charging current of the large shunting condenser 24; For high input levels to the control tube 1 there will be only slight ripples in this plate current, but near the operating level considerable variations will exist. However, the plate current flow in the variable impedance tube during the continuous application of speech currents to the control tube 1 will be such as to maintain a low impedance between the cylindrical grid l6 and the cathode I4 of the variable impedance tube I3 and thus a high loss in the transmission path EA.

' When the application of speech voltages to the control tube! ceases, first the control tube 1 and then the variable impedance tube l3 will return to the normal inoperative condition causing the high loss in the path EA to be removed. By proper selection of the size of the condenser 24 and associated elements, any desired hangover in operation of the suppressor may be obtained. The amount of hangover required will depend ,on the location of the suppressor in the four-wire circuit. If the suppressors are connected at the mid-point of the four-wire circuit, a large amount of hangover will be required to prevent premature release whereas if the echo suppressors are located at or near the terminals of the fourwire circuit, little or no hangover is necessary.

It will be noted that there is no battery in the circuit connecting cylindrical grid l6 and the cathode M of the variable impedance tube l3 across the path EA and hence the current flowing in that circuit when the variable impedance tube 13 is operated is very small. It has been found that this current during operation of the suppressor is so small that click disturbances in the receiving circuits associated. with the path EA and WA are substantially eliminated.

The disabler portion of the echo suppressor operates as follows: Let it be assumed that speech currents to be transmitted from east to west are initiated in the path EA, and, at the time they reach the point therein where the input of the disabler is connected,no west to east speech currents have reached the input of the suppressor associated with the path WA. The speech volt'-' ages diverted from the path EA into the disabler are selectively amplified in the input circuit thereof by special amplifier 30 and amplifier 3|, and are'then impressed on the input of the threeelectrode gas-filled disabler tube 34 by trans former 33. When the impressed speech voltages reach a sufficiently high level, ionization will take place in the tube 34 and a small current will flow. in its plate circuit. This current flows through the resistance 42 in the spiral grid cir-= c'uit'of the variable impedance tube' l3 and the IR drop 'in resistance 42 adds a negative bias to the total bias of the spiral grid sufficient to hold the variable impedance tube l3 unoperatedeven thoughthe control tube 1 subsequently operates since when the variable impedance tube is in'the de-ionized conditions the spiral grid has a greater control over the ionization than does the cylindrical grid. Therefore, the action of the suppressor is prevented since its operation depends upon the ionization of the variable impedance tube l3;

Thevariable impedance tube I3 is placed in the circuit in such manner that the spiral grid ll thereof loses control when ionization takes place in the tube' and therefore any. impulse which arrives from the disabler circuit after ionization has already taken place in the variable impedance tube cannot affect its operation. False disabling of the suppressor by too quick action of the disabler on initial echoes is prevented by the slowing up of the effective disabling potential on spiral grid ll provided by the choke coil 43 and the resistance condenser filter 45, but if the disabler has built'up a voltage drop across the condenser in filter 45 the normal available plate voltage is not suflicient to cause ionization in the variable impedance tube l3.

The echo supressor circuit shown in Fig. 2 differs from the echo suppressor circuit of Fig. 1 just described merely in the construction of the variable impedance elementand the control circuit for controlling that element, the other parts of the circuits of Fig. 2 being identical with the corresponding parts in the circuit of Fig. 1 as indicated by the use of the same reference characters for designating them.

The variable impedance device 46 comprises two four-electrode gas-filled tubes 47 and 48, similar to the tubes l and E3 of the system of Fig. 1, and the bridging transformer 49. The tube 41 comprises a cathode 55, a plate or anode 5!, a cylindrical grid 52 located between the cathode and anode, and a spiral grid 53 located between the cathode and the cylindrical electrode and very close to the cathode. Similarly, the gasfilled tube 48 comprises a cathode 54, a plate or anode 55, a cylindrical grid 56 located between the cathode and anode and a spiral grid 51 located between the cylindrical grid and the oathode and very close to the cathode. The plates 5|, of tubes 41 and 48 are connected through the equal resistances 58 and 5S.

The cathode-cylindrical grid circuits of the tubes 4? and 48 are connected in push-pull relationship. The cylindrical grid-cathode circuit of tube 41 comprises the parallel resistancecondenser combination fiz and the upper half of the primary (high) Winding 53 of the step-down bridging transformer 49, and the cylindrical gridcathode circuit for tube 48 comprises the parallel condenser-resistance combination B4 and the lower half'of the primary winding 53 of transformer 49. The secondary (low) winding 65 of bridging, transformer 49 is; connected across the path EA at the points 6.6; 61 in the output of a repeater (notshown) in that path. The bridg-.

impedance of that path for that purpose as shown inthe'system of Fig. 1. Since the loss in the path EA during transmission of speech currents has to be kept small and substantially constant in the speech frequency range, the open-circuited impedance of the transformer 49 'is made high. In order to increase the maximum loss during suppression, the resistance of the transformer windings is made low and the turns ratio high for stepping down the im- I pedance of the tubes.

Ihe control circuit comprises three gas-filled tubes, i-. e., the operator tube 68, the starter tube 69 and the stopper tube 10. The operator gas-filled tube 68 is a three-electrode tube similar to the gas-filled disabler tube 34 in the system of Fig. l. The ca-thode 'II and cylindricalgrid 7 13- of the tube 68' are coupled to the output of the suppressor amplifier tube 2 by transformer 6. The cylindrical grid 13 is negatively polarized by the battery 14. Plate current is supplied to the plate 12 of tube 88 through the resistance 15 from plate battery 16. A condenser 11 anda resistance 18 are connected in series directly across the plate 12' and the cathode ll of the tube 68. A resistance I9 is connected between the negative side of plate battery 16 and the cathode H of the tube 68. The resistances 15 and 19 are very high (substantially 100,000 ohms each) so that the current flowing in tube 68 when the ionization takes place therein is very small. The gas-filled starter tube 69 comprises four electrodes, a cathode a plate 8|, a cylindrical grid 82 connected between the plate and cathode, and a spiral grid 83 connected between the cylindrical grid and the cathode and very near the cathode.

The gas-filled stopper tube 10 is similar to the operator tube 68 comprising three electrodes, a cathode 84, a plate or anode 85 and a cylindrical grid 86'.

Space current is supplied from the plate battery 81 to'the plate of the starter tube 69 through the resistance'88, to the plate of the-stopper tube 10 through the resistance 89,,and to the plates Y 88 in series.

The cylindrical grid 82 of the starter tube 69 is negatively biased by battery 9| so as to be normally in the de-ionized condition. The cylindrical grid-cathode circuit of the starter tube includes between the cylindrical grid 82 and the cathode 80'the battery 9! and the resistance 19. The cylindrical grid 86 of the stopper tube 10 is negatively biased by the grid battery but is con- ;nected to the positive pole of the plate battery 16 through the grid-biasing battery and high resistance '15 so that it has a resultant positive bias and is normally in the ionized condition. The T spiral grid 83 of the starter tube '69is negatively biased by. the battery 93 and is connected to the positive pole of battery 16 through resistance 94 so that normally the spiral grid 83 has a small positive bias. The suppressor portionof the echo suppressor of Fig. 2 operates as follows: Let it beassumed L that the transmission paths EA and WA are in their normal operativecondition, and that speech waves for transmission from east to west are in itiated in the transmission patlrWA. A portion of these speech waves will be diverted into the input circuit of thevsuppressor and will be selectively amplified in the input circuit thereof. The resultant amplified speech voltagesare impressed through the transformer 6 on the cylindrical gridcathode circuit of the gas-filled operator tube 68. 2 When the positive peak of the impressed speech voltage exceeds a given value, the operator tube will ionize and current will be established in the high resistances l5 and 19 in the plate circuit 0 25 the tube.

Before ionization starts in the operator tube 68, the condenser 11 is charged to the voltage of the plate battery 16, but when the tube ionizes due to the, applied positive peak speech voltage, the condenser 11 must discharge through the resistance 18 and'the tube to a voltage equalto the voltage drop across the tube. The resistance 18 is small so that the initial current flowing therein and hence in the. tube will be high and will diminish in a regular exponential manner. During the discharge of the condenser Tl, the very low value of current flowing through the high resistances 'I5and 19 will also be passing through the tube 68 quite independent of the discharge current therethrough from the condenser 11, since the voltage drop across the tube is independent of current. During the time of the condenser discharge the total currents of the tube will be high and,ptherefore, a higher negative grid current (current from cathode to grid within the tube 68) will berequired to stop ionization. The current in resistances 15 and 19 remains constant until the current from the discharge condenser 11 decreases to such a value that the negative voltage swing of the input current becomes sufficient to stop ionization as indicated by the first abrupt change in the current after the tube first ionizes. As soon as the ionization stops the condenser TI is charged up again by current from the battery 16. This charging tends to maintain the current in the resistances l5 and I9. The condenser voltage builds up to a certain value determined by the positive peak in the output voltage which is suificient to start ionization again. This process is continued over and over during the time speech waves are impressed upon the input circuit of the suppressor. With increasing levels of the impressed speech waves the condenser voltage at which the operator tube 68 goes out, that is, is rendered inoperative'becomes higher with the result that this process of starting and stopping of the ionization becomes much more rapid as the input level is increased. The total efiect is to give a fairly steady unidirectional current in the resistances 15 and 19. This steady current is used as asource of grid-biasing voltage for the following starter and stopper tubes. 7

When the input voltage to the operator tube 68 7 is removed and ionization stops, current will continue to flow in the resistances 15' 19 and 19 during the period required for the condenser 11 to be charged up to the voltage of the plate battery 16. By proper selection of the value of the condenser H and the series resistance 18, therefore, the suppressor may be given any desired hangover in operation.

When the operator tube 63 ionizes, the grid potentials of the stopper and starter tubes are reversed in polarity due to the IR drops in the resistances "i5 and it. Because of the positive bias supplied to the grid of the starter tube it, that tube will immediately ionize. Though the resultant negative bias on the grid of the stopper tube 19 is not sufiicient to stop ionization therein (which depends upon the plate current), the action of the condenser 9% connected between the plate 85 of the stopper tube 79 and the plate 8! of the starter tube 69 will practically instantaneously stop ionization in the stopper tube.

Normally the condenser 08 is charged to a voltage of a value which is equal to the difference between the normal voltage above ground of the plate 8i of the starter tube and the lower normal voltage above ground of the plate 85 of the stopper tube. When the starter tube 59 ionizes, the potential of the terminal of the condenser 9i! connected to its plate BI is suddenly brought to a potential equal to the voltage drop of the starter tube above ground. The other terminal of the condenser 90 which is connected to the plate of the stopper tube 70 must undergo an instantaneous change of the same magnitude on account of the high transient impedance of the external circuit compared with that of the condenser This makes the plate potential of the stopper tube 10 sufficiently negative with respect to the potential of its cathode to bring about de-ionization in that tube which condition is maintained by the negative input bias. will then charge up to a voltage equal to the difference between the value of the plate battery 81 and the voltage drop of the starter tube 69, in the reverse direction. When the operator tube 68 is returned to normal, that is, to the inoperative condition, the polarities of the potentials applied to the grids of the starter and stopper tubes will be again reversed and the stopper tube l will be ionized, whereupon the action of the condenser 99 as before will stop ionization of the starter tube 69 and the entire circuit will return to normal.

In this circuit either the starter tube 59 or the stopper tube 70 will be continuously ionized during the time in which speech voltages are being impressed upon the operator tube 68. It should also be noted that this method of stopping the ionization will be effective theoretically for all values of plate current.

The IR drop in the resistance 88 when the starter tube 69 ionizes, becomes the plate voltage of the variable impedance tubes 41 and 40. The spiral grid 53 of variable impedance tube 47 and the spiral grid 51 of variable impedance tube 48 also become positive with respect to the cathodes of these tubes when this IR drop across resistance 88 is supplied with the result that current flows in the tubes 4'! and 48. The value of this current is limited only by the value of the resistances 58 and 59 connected in series between the plates of the variable impedance tubes. When current flows in the variable'impedance tubes 41 and 48, a low impedance path is established between the cathode and cylindrical grid of each tube. The primary winding 63 of the bridging The condenser 90 transformer 49 is thus effectively short-circuited through the low impedance paths within tubes 47 and 48. As stated before, the secondary (low impedance) winding 65 of transformer 49 is connected directly across the transmission path EA at the points 66 and 61. The impedance across path EA, therefore, as the result of operation of the variable impedance tubes 4? and it, is reduced to such an extremely low value that the path EA is effectively short-circuited. Thus, any echoes or reflected currents which may reach the bridging point in that path while the variable impedance tubes 4? and 48 are maintained operated in response to the speech waves from the path WA are substantially suppressed.

When the speech waves from the path WA cease, the operator tube 68, the starter tube 69 and the stopper tube return to their normal conditions with the result that the flow of space current in the variable impedance tubes 41 and 48 is stopped and the normal high impedance shunt across the path EA provided by the bridging transformer 49 is re-established making that path again operative to transmit efficiently.

The negative voltages applied normally to the spiral grids 53 and 5'! of tubes 41 and 48, respectively, due to the grid-biasing batteries 60 and GI, respectively, are made high enough to prevent the setting up of ionization between the cathode and cylindrical grid of either tube by the normally transmitted speech in the path EA. The condensers in the parallel-condenser-resistance combinations 62 and 64 in the output circuits of the variable impedance tubes are made of such values as to neutralize the leakance inductance of the bridging transformer 49 and bring the point of maximum loss across the path EA to the frequency where the greatest power in speech lies, which is at about 1000 cycles. The resistances in the combinations 62 and 64 supply a direct current path between the grids and their respective cathodes. These resistances may be made of such high value that they will not affect the tuning because of the extremely low direct current flowing in the output circuits of the variable 1mpedance tubes 41 and 48.

vThe operation of the disabler circuit will now bejdescribed. It will be assumed that speech waves are being transmitted over the path EA in the direction from west to east and at the time at which these waves arrive at the point in the path EAv where the disabler is connected, no east to west speech waves have arrived at the point in the path WA where the input of the suppressor is connected.

The speech waves diverted into the disabler circuit are selectively amplified in the input circuit thereof and after a sufiiciently high level has been reached, the speech voltages impressed upon the input of the disabler tube 34 will cause ionization in that tube resulting in a small current flow in the plate circuit thereof. Before the tube 34 ionizes, the spiral grid 83 of the starter tube 69 has a normal positive bias equal to the difference between the positive voltage of the battery 16 and the negative bias provided by the biasing battery 93. After ionization starts in the disabler tube 34, the current flowing in the resistance 94 in the output circuit of that tube causes a negative bias equal to the voltage drop in that resistance to be added to the total bias of the spiral grid of the starter tube. By proper selection of the value of the resistance 94, this negative bias may be made such as will hold the starter tube 6.9 unoperated even though the operator-tube 68 subsequently operates, since the spiral grid 83 of the tube 69 has a greater control over the ionization therein than does the cylindrical grid 82. Therefore, after the dis abler tube 34 has been once "operated in response to speech waves received from the path EA, action of the suppressor portion of the circuit is prevented since the operation of the last tubes therein, variable impedance tubes 41 and 48, depends upon the ionization of the starter tube. However, if the starter tube 69 is operated in response to speech waves from the path WA prior to operation of the'disabler tube 34 in response to speech waves from the path EA, the operation of the suppressor will not be interrupted since the negative bias appliedthrough resistance 94 to the spiral grid 83 of the starter tube will'not be sufficient to stop the high ionization current in that tube.

It is usually desirable to have the suppressor and the disabler remain operated for about second after the inputs thereto have been re- 7 moved, to allow for the suppression of weak endings of syllables in speech and echoes. The

- desired hangover time in the operation of the suppressor portion of the circuit is obtained by giving the proper value to the condenser 11 and series resistance 18 in the output 'of the operator tube 68. As was explained above, the condenser 17 charges up through the resistances 15, I9 and 18 when ionization in the operator tube 68 stops.

' Similarly, when the input speech voltage is removed from the operator tube 68, the current therein stops and the condenser 11 is charged up by current from the plate battery 16 and od'e in the disabled tube 34.

There are certain variations in the hangover timeof the suppressor both at fixed and also at different levels. Asmight be expected from the previous explanation of the action of the condenser ll, the hangover at a constant input depends upon the charge of the condenser 11 at the 'moment the input to the operator tube 68-is removed. By making the normal negative bias on the cylindrical grid 13 of the operator tube 68 high, the variations in the condenser voltage can be kept within narrow limits if the input level exceeds the minimum operating level by about one decibel.

After the voltage on the operator tube has reached a given value, the establishment of the loss in the echo path is practically instantaneous.

The time of operation of the disabler will be only slightly longer than that of the suppressor.

The retardation'coil 43 in the output of the disabler tube 34 and the associated shunting condenser and resistance 95 act to slow. up the initial current in the plate circuit of the disabler tube and hence make the operation of the disabler slightly longer. The purpose of this slowing up of the disabler operation is to insure the operation of the suppressor 'before that of the disabler'if'the same voltage is applied simul- .taneously to both of them so as'to prevent the initial echoes of incoming speech from falsely operating the disabler. Thetime of operation of the disabler obviously need not be very fast.

It will be noted that there is no battery in the cylindrical grid-cathode mesh of the variable impedance tube 47 or of the variable impedance tube as and hence the current'which will flow through the primary winding of the bridging transformer '49 when the suppressor is operated is very small. Vhth such a circuit, the click is reduced to a minimum since the initial currents 1 through the condensers in the combinations 62 and ii i are small. However, with some tubes it may be advantageous to insert a small battery in the cylindricalgrid-cathode meshes of the variable impedance tube to reduce the click or shift 1 the characteristic.

For simplification, only one of the pair of echo suppressors which are usually associated with the four-Wire transmission circuit has been illustrated in the systems of Figs. 1 and 2. It is to be understood that another echo suppressor identical with that illustrated would be provided to suppress echoes when the transmission is from west to east. The input of the suppressor portion of the latter echo suppressor would be .con- 2 nected to the path EA and the disabler portion is changed to a low impedance in response to 40' establishment of direct current flow between certain of its electrodes may be utilized in place of the devices illustrated and described.

In the circuit of Fig. 2, the variable impedance device 66 has been shown as comprising two four-element gas-filled tubes connected in push-pull relation as it was found that some-. what better results from the standpoint of elimination of click disturbances was obtained by the use of the push-pull tubes. However, if 5 the variable impedance device 48 comprises a single four-electrode tube connected in, a manner similar to that of either push-pull tube illustrated, the single tube will produce a substantial reduction in click disturbances inasmuch as no plate battery would be used in the cylindrical-grid-cathode mesh thereof.

In the circuits'illustrated and described, each of the various tubes have been shown as utilizing a heater type cathode. For simplification, the 0 details of. the cathode circuits have not been illustrated. Of course, any other type of cathode may be used in place of those shown. Other. modifications which may be made in the circuits of the invention without departing from the spirit and scope thereof as defined in the appended claims will occur to persons skilled in the art.

What is claimed is:

1. In combination, a transmission circuit, a source of alternating current signals, a normally unoperated gas-filled electric discharge device 7 having a cathode, an anode, a grid electrode located between said cathode and said anode, and circuits therefor, the normal high grid-cathode impedance of said device being coupled effectively in shunt with said transmission circuit, means responsive to the alternating current signals received from said source to apply a voltage between said cathode and said anode to cause a flow of direct current between said cathode and said anode, of sufficient amplitude to reduce the impedance of the path between said cathode and said grid electrode and thus said shunt impedance to a low value and means for maintaining said flow of direct current for a time interval at least equal to the duration of said signals.

2. The combination of claim '1 and in which the said means for causing said flow of direct current comprises a second electric discharge device having an input and an output circuit, said output circuit and the cathode-anode circuit of the first electric discharge device being connected in series, means for impressing said received alternatingcurrent signals on said input circuit so as to cause operation of said second device to initially establish a fiow of direct current of the required amplitude in the series circuit, and said means for maintaining said flow of direct current between the cathode and anode of said first device comprises means associated with said second discharge device for maintaining said current flow in the cathode-anode circuit of said second device, at least as long as said alternating current signals are impressed on said input circuit.

3. The combination of claim 1 and in which the said means for applying a voltage between said cathode and anode comprises a second gasfilled electric discharge device having a cathode, an anode, a control grid therebetween, a second grid electrode located between said control electrode and said anode and connected directly to the cathode, and circuits therefor, a resistance connected directly between the cathode and anode of the first device, the cathode-anode circuits of the two discharge devices being connected in series, a source of plate current in said series circuit, means for impressing said alternating current signals on the cathode-control grid circuit of said second device so as to cause operation of said second device to initially produce a flow of direct current through said resistance, and said means for maintaining said flow or direct current between the cathode and anode of the first device comprises a condenser in series with a small resistance shunted directly between the anode and cathode of said second device, said condenser being normally charged from said plate current source and discharging through said small resistance and said second device when said second device is operated.

4. The combination of claim 1 and in which the said means for applying a voltage between said cathode and said anode comprises a second gasfilled electric discharge device having a cathode, an anode, a control grid therebetween, a second grid electrode located between said control electrode and said anode and connected directly to the cathode, and circuits therefor, a resistance connected directly between the cathode and anode of the first device, the cathode-anode circuits of the two discharge devices being connected in series, a source of plate current in said series circuit, means for impressing said alternating current signals on the cathode-control grid circuit of said second device so as to cause operation of said second device to initially produce a flow of direct current through said resistance,

and said means for maintaining said flow of direct current between the cathode and anode of the first device comprises two parallel combinations shunted directly between the cathode and anode of said second device, one combination comprising a large condenser in series with a resistance and the other combination comprising a smaller condenser in serieswith a smaller resistance, the condensers being normally charged from said plate current source and being discharged through the series resistances and said second device when said second device is operate-d.

5. In combination, a transmission circuit, a source of alternating current signals, a normally unoperated gas-filled electric discharge device having a cathode, an anode, a control gri therebetween and a second grid electrode located between the control grid and the anode, the normal high impedance between said cathode and said second grid electrode being coupled effectively in shunt with said circuit, means for normally main: taining said control grid at a given negative potential with respect to said cathode, means responsive to alternating current signals received from said source to apply a voltage between said cathode and said anode to cause a flow of direct current therebetween suflicient to reduce the impedance of the space path between said cathode and second grid electrode and thus the impedance shunting said transmission circuit to a low value, and means connected to said transmission circuit and responsive to transmission of alternating current signals therein to prevent the establishment of a flow of direct current between said cathode and said anode in response to alternating current signals subsequently received from said source.

6. The combination of claim 5 and in which thevalue of the negative potential at which said control grid is normally maintained is suflicient to prevent ionization in the gas path between said cathode and said second grid electrode in response to alternating current signals transmitted over said transmission circuit.

7. The combination of claim 5 and in which the last mentioned means comprises means for applying sufficient negative bias to the control electrode of said device to prevent the establishment of ionization therein by subsequently received alternating current signals from said source.

8. In common, a transmission circuit, a source of alternating current signals, a normally unoperated gas-filled electric discharge device having a cathode, an anode, a control grid located therebetween and a second grid electrode located between said control grid and said cathode, the normal high impedance between said second grid electrode and said cathode being coupled effectively in shunt with said transmission circuit, means responsive to alternating current signals received from said source to apply a voltage between said cathode and said anode to cause a flow of direct current therebetween in sufiicient amount to reduce the impedance between said second grid electrode and said cathode and thus the shunt impedance across said circuit to a low value, means responsive to transmission of alternating current signals in said transmission circuit for applying a negative biasing voltage to said control grid of such value as to prevent the establishment of a flow of direct current between said cathode and anode in response to alternating current signals subsequently received from said source, but-10f insufiicient" value to cause de.- ionization in said device if flow of direct current. between said cathode and anode has already been established in response to alternating current-signa'ls previously received from said source.

9; Thecombination of claim 1 and in whichthe said means for causing and maintaining a flow of direct current between said cathode andsaid anode'com-prises a second and a third'gas-filled electricdischarge device each having a cathode, an anode, a grid electrode and circuits. therefor; the cathode-grid circuits of said second and third devices and the cathode-anode circuitsthereof, respectively being connected in pushpull relationship, the cathode-anode circuit of said second device and of the first gas-filled devicebeing connected in series so that ionization insaid second device establishes a flow of direct 7 current between the'anode and cathode of said first device, a condenser connected directly be tween the anodes of said second and third devices, said second device being normally de-ionized and. said third device being normally in; the: ionized condition, a fourth electric discharge device. supplied. with alternating current signalsr' from said source and operatively responsive tax the positive peaks of the signal impulses to apply a. positive; bias to the grid'electrode of said, second device to cause ionization therein and simultaneously to apply a negative bias to the grid electrode of said third device to cause de-ioniza-- tion therein and responsive to cessation in theapplied signal impulseto return said secondand: third devices to their normal conditions, and means associated with said fourth electric discharge device for causing: this operation to-berepeated in response. to each'succeeding impulse of the applied signals so as to maintain said di-- rect current flow between the. cathode andanode of said first device at least as long assai'd signals are substantially continuously applied to said fourth electric discharge device.

BJORN G. BJORNSON. V

QUENTIN E. GREENWOOD 

